| 摘要 |
<p>PURPOSE:To eliminate a choke coil and to miniaturize a circuit and to package the circuit with high density by providing an electric current limit circuit function to give the electric potential difference between base emitters of a transistor for a direct current limit with the voltage drop of an electric current detecting resistance. CONSTITUTION:At a trunk line trunk COT, an alternate current component (sound signal) on a telephone line L is inputted through a capacitor C to an exchanging switch SW. A choke circuit CH does not pass an alternate current component, passes only a direct current component and forms a direct current loop. The direct current resistance of the choke circuit CH is R=RB/hfe+Rs, an alternate current impedance is RB+iwC0, and when an electric current amplifying ratio hfe is made larger, a base resistance RB comes to be also larger, and therefore, the alternate current impedance can be made larger without enlarging the direct current. Since the electric current limit circuit is formed by a transistor Q3 and a resistance Rs, the direct current between electric current paths 1 and 1a is limited.</p> |
